פועל משלו הוא אוכל (Bava Metzia 92a)

The Gemara says that the נ"מ is whether or not he can give to his wife and children. The Tur raises the question, what about if he picks it up to eat himself and then decides to give it to his wife or children (even if משמים הוא אוכל)? The Tur has 2 opinions and rejects the opinion that it is permitted. The Gra on Shulchan Aruch says that the 2 opinions are based on a machlokes Rashi and Tosafos. Rashi says "כל זמן דלא מטי לידיה לא זכי ליה". The implication is that once he does pick it up it his to do what he wants with including giving to his wife and kids. Tosafos however, says that he is not זוכה until he is "לועס ואוכל" which means that he cannot give it to his wife and kids.

In truth, Rashi on ד"ה אינו קוצץ) עמוד ב ) seems to say like Tosafos "דאין לפועל קטן זכות אלא שנותן לתוך פיו"

Also, in truth, Tosafos is a bit of a פלא, why should he only be זוכה when he puts it in his mouth? Why should this be different then any other מתנות עניים?

אתנן אסרה תורה (Bava Metzia 91a)

The way we have the גירסא the מסקנה is that there is no חיוב that ב"ד can impose because of קם ליה בדרבה מיניה however there is a חיוב בבא לצאת ידי שמים.

The מאירי in בבא קמא quotes a number of Rishonim who have a different גירסא in the Gemara and come to the opposite conclusion. They learn that both by אתנן and חסימה there is a חיוב to pay that ב"ד can enforce. What happened to קם ליה בדרבה מיניה? They learn that קם ליה בדרבה מיניה does not apply to a התחייבות מדעת. It only applies to a חיוב that is based on an action not a person's דעת. It comes out that according to these Rishonim by חסימה you would not say קם ליה בדרבה מיניה because the renter of the פרה agreed to the חיוב מדעת.

חסמה בקול (Bava Metzia 90b)

The simple understanding of the Gemara is like Rashi that he causes the animal not to eat by his voice by yelling at the animal when it is about to eat.

However, if you look carefully at Tosafos (ד"ה רבי יוחנן), you will see that Tosafos disagrees. Tosafos is bothered by a question that רבי יוחנן himself holds that עקימת שפתיו is not a מעשה by נשבע מימר ומקלל and therefore what is the חילוק here and why should חסימה בקול be חייב? Tosafos answers that here he is causing the animal to be דש with his voice and therefore בדיבוריה קא עביד מעשה. Tosafos learns pshat in חסמה בקול that he muzzled the animal normally and then with his voice caused the animal to be דש. This is very difficult in the words of the Gemara, according to Tosafos the Gemara should have asked דשה בקול. He wasn't חוסם בקול here at all. Tosafos is forced into this pshat because they need his דיבור to actually do something, cause the עבירה to happen. By לא תחסם the עיקר עבירה is the דישה while the animal is muzzled and therefore they had to learn that his דיבור caused the דישה.